Methods: This proposal is to use the Intermediate Voltage Electron Microscopy (IVEM) - Tandem Facility at the Argonne National Laboratory to conduct in-situ TEM study of the microstructural evolution of NaMnO2 under ion irradiation doses of 0, 1, 3, 5, 10 dpa at both room temperature and 300 C. The NaMnO2 is an important sodium-ion battery cathode material. The defect chemistry will be further characterized using advanced synchrotron X-ray spectroscopy and imaging techniques that are sensitive to the local defect chemical and structural environments. The experimental results will also be used to inform atomistic modeling for constructing appropriate defect models and validate modeling results. Potential impact: The proposed research aims to develop a knowledge base for designing next-generation rechargeable batteries for operations under extreme conditions including irradiation. This knowledge will be extremely important for understanding and improving the performance of battery-powered devices and sensors in radiation environment. Radiation-induced defects can lead to complex microstructural evolution in the cathode oxide materials and may degrade the battery performance. On the other hands, defects may serve as ionic transport carriers and channels and thus enhance the battery performance. The true role of defects on the battery performance is unknown. The cathode material (NaMnO2) has multiple phase transformations upon charging and discharging, which create internal stress and slow down the sodium ion kinetics. Radiation may affect such phase transformation behavior in NaMnO2, either suppress or enhance the phase transformations. For the work performed at the IVEM, we plan to directly observe the radiation-induced microstructural evolution in NaMnO2. If successful, we will elucidate the role of radiation on the phase transformation in NaMnO2. To our best of knowledge, such study has never been conducted previously. The performance period is expected to be 5 days. Anticipated scientific outcome: The anticipated scientific outcomes include: (1) understanding of the phase stability of NaMnO2 at different irradiation doses and temperatures, (2) in situ observation of ion irradiation induced defect and microstructure evolution, (3) quantification of defect type, size, density, and distribution with irradiation dose and temperature, (4) correlation between phase transformation tendency and irradiation dose, (5) new understanding of defect and microstructure evolution in NaMnO2 through a combined experimental and modeling approach.